Allow function arguments as base classes (#14135)

Fixes #5865 

Looks quite easy and safe, unless I am missing something. Most changes
in the diff are just moving stuff around.

Previously we only applied argument types before type checking, but it
looks like we can totally do this in semantic analyzer. I also enable
variable annotated as `type` (or equivalently `Type[Any]`), this use
case was mentioned in the comments.

This PR also accidentally fixes two additional bugs, one related to type
variables with values vs walrus operator, another one for type variables
with values vs self types. I include test cases for those as well.

mypy/checker.py

@@ -184,7 +184,6 @@
     LiteralType,
     NoneType,
     Overloaded,
-    ParamSpecType,
     PartialType,
     ProperType,
     StarType,
@@ -203,14 +202,14 @@
     UnboundType,
     UninhabitedType,
     UnionType,
-    UnpackType,
     flatten_nested_unions,
     get_proper_type,
     get_proper_types,
     is_literal_type,
     is_named_instance,
     is_optional,
     remove_optional,
+    store_argument_type,
     strip_type,
 )
 from mypy.typetraverser import TypeTraverserVisitor
@@ -1174,30 +1173,8 @@ def check_func_def(
                             if ctx.line < 0:
                                 ctx = typ
                             self.fail(message_registry.FUNCTION_PARAMETER_CANNOT_BE_COVARIANT, ctx)
-                    if typ.arg_kinds[i] == nodes.ARG_STAR:
-                        if isinstance(arg_type, ParamSpecType):
-                            pass
-                        elif isinstance(arg_type, UnpackType):
-                            if isinstance(get_proper_type(arg_type.type), TupleType):
-                                # Instead of using Tuple[Unpack[Tuple[...]]], just use
-                                # Tuple[...]
-                                arg_type = arg_type.type
-                            else:
-                                arg_type = TupleType(
-                                    [arg_type],
-                                    fallback=self.named_generic_type(
-                                        "builtins.tuple", [self.named_type("builtins.object")]
-                                    ),
-                                )
-                        else:
-                            # builtins.tuple[T] is typing.Tuple[T, ...]
-                            arg_type = self.named_generic_type("builtins.tuple", [arg_type])
-                    elif typ.arg_kinds[i] == nodes.ARG_STAR2:
-                        if not isinstance(arg_type, ParamSpecType) and not typ.unpack_kwargs:
-                            arg_type = self.named_generic_type(
-                                "builtins.dict", [self.str_type(), arg_type]
-                            )
-                    item.arguments[i].variable.type = arg_type
+                    # Need to store arguments again for the expanded item.
+                    store_argument_type(item, i, typ, self.named_generic_type)
 
                 # Type check initialization expressions.
                 body_is_trivial = is_trivial_body(defn.body)

mypy/semanal.py

@@ -273,6 +273,7 @@
     get_proper_types,
     invalid_recursive_alias,
     is_named_instance,
+    store_argument_type,
 )
 from mypy.typevars import fill_typevars
 from mypy.util import (
@@ -1315,7 +1316,10 @@ def analyze_function_body(self, defn: FuncItem) -> None:
             # Bind the type variables again to visit the body.
             if defn.type:
                 a = self.type_analyzer()
-                a.bind_function_type_variables(cast(CallableType, defn.type), defn)
+                typ = cast(CallableType, defn.type)
+                a.bind_function_type_variables(typ, defn)
+                for i in range(len(typ.arg_types)):
+                    store_argument_type(defn, i, typ, self.named_type)
             self.function_stack.append(defn)
             with self.enter(defn):
                 for arg in defn.arguments:
@@ -2018,7 +2022,9 @@ def analyze_base_classes(
                 continue
 
             try:
-                base = self.expr_to_analyzed_type(base_expr, allow_placeholder=True)
+                base = self.expr_to_analyzed_type(
+                    base_expr, allow_placeholder=True, allow_type_any=True
+                )
             except TypeTranslationError:
                 name = self.get_name_repr_of_expr(base_expr)
                 if isinstance(base_expr, CallExpr):
@@ -6139,7 +6145,11 @@ def accept(self, node: Node) -> None:
             report_internal_error(err, self.errors.file, node.line, self.errors, self.options)
 
     def expr_to_analyzed_type(
-        self, expr: Expression, report_invalid_types: bool = True, allow_placeholder: bool = False
+        self,
+        expr: Expression,
+        report_invalid_types: bool = True,
+        allow_placeholder: bool = False,
+        allow_type_any: bool = False,
     ) -> Type | None:
         if isinstance(expr, CallExpr):
             # This is a legacy syntax intended mostly for Python 2, we keep it for
@@ -6164,7 +6174,10 @@ def expr_to_analyzed_type(
             return TupleType(info.tuple_type.items, fallback=fallback)
         typ = self.expr_to_unanalyzed_type(expr)
         return self.anal_type(
-            typ, report_invalid_types=report_invalid_types, allow_placeholder=allow_placeholder
+            typ,
+            report_invalid_types=report_invalid_types,
+            allow_placeholder=allow_placeholder,
+            allow_type_any=allow_type_any,
         )
 
     def analyze_type_expr(self, expr: Expression) -> None:
@@ -6188,6 +6201,7 @@ def type_analyzer(
         allow_param_spec_literals: bool = False,
         report_invalid_types: bool = True,
         prohibit_self_type: str | None = None,
+        allow_type_any: bool = False,
     ) -> TypeAnalyser:
         if tvar_scope is None:
             tvar_scope = self.tvar_scope
@@ -6204,6 +6218,7 @@ def type_analyzer(
             allow_required=allow_required,
             allow_param_spec_literals=allow_param_spec_literals,
             prohibit_self_type=prohibit_self_type,
+            allow_type_any=allow_type_any,
         )
         tpan.in_dynamic_func = bool(self.function_stack and self.function_stack[-1].is_dynamic())
         tpan.global_scope = not self.type and not self.function_stack
@@ -6224,6 +6239,7 @@ def anal_type(
         allow_param_spec_literals: bool = False,
         report_invalid_types: bool = True,
         prohibit_self_type: str | None = None,
+        allow_type_any: bool = False,
         third_pass: bool = False,
     ) -> Type | None:
         """Semantically analyze a type.
@@ -6260,6 +6276,7 @@ def anal_type(
             allow_param_spec_literals=allow_param_spec_literals,
             report_invalid_types=report_invalid_types,
             prohibit_self_type=prohibit_self_type,
+            allow_type_any=allow_type_any,
         )
         tag = self.track_incomplete_refs()
         typ = typ.accept(a)

mypy/stubtest.py

@@ -354,7 +354,7 @@ def _verify_final(
 ) -> Iterator[Error]:
     try:
 
-        class SubClass(runtime):  # type: ignore[misc,valid-type]
+        class SubClass(runtime):  # type: ignore[misc]
             pass
 
     except TypeError:

mypy/treetransform.py

@@ -550,7 +550,7 @@ def visit_super_expr(self, node: SuperExpr) -> SuperExpr:
         return new
 
     def visit_assignment_expr(self, node: AssignmentExpr) -> AssignmentExpr:
-        return AssignmentExpr(node.target, node.value)
+        return AssignmentExpr(self.expr(node.target), self.expr(node.value))
 
     def visit_unary_expr(self, node: UnaryExpr) -> UnaryExpr:
         new = UnaryExpr(node.op, self.expr(node.expr))

mypy/typeanal.py

@@ -201,6 +201,7 @@ def __init__(
         allow_param_spec_literals: bool = False,
         report_invalid_types: bool = True,
         prohibit_self_type: str | None = None,
+        allow_type_any: bool = False,
     ) -> None:
         self.api = api
         self.lookup_qualified = api.lookup_qualified
@@ -237,6 +238,8 @@ def __init__(
         # Names of type aliases encountered while analysing a type will be collected here.
         self.aliases_used: set[str] = set()
         self.prohibit_self_type = prohibit_self_type
+        # Allow variables typed as Type[Any] and type (useful for base classes).
+        self.allow_type_any = allow_type_any
 
     def visit_unbound_type(self, t: UnboundType, defining_literal: bool = False) -> Type:
         typ = self.visit_unbound_type_nonoptional(t, defining_literal)
@@ -730,6 +733,11 @@ def analyze_unbound_type_without_type_info(
                 return AnyType(
                     TypeOfAny.from_unimported_type, missing_import_name=typ.missing_import_name
                 )
+            elif self.allow_type_any:
+                if isinstance(typ, Instance) and typ.type.fullname == "builtins.type":
+                    return AnyType(TypeOfAny.special_form)
+                if isinstance(typ, TypeType) and isinstance(typ.item, AnyType):
+                    return AnyType(TypeOfAny.from_another_any, source_any=typ.item)
         # Option 2:
         # Unbound type variable. Currently these may be still valid,
         # for example when defining a generic type alias.

mypy/types.py

@@ -7,6 +7,7 @@
 from typing import (
     TYPE_CHECKING,
     Any,
+    Callable,
     ClassVar,
     Dict,
     Iterable,
@@ -29,6 +30,7 @@
     ArgKind,
     FakeInfo,
     FuncDef,
+    FuncItem,
     SymbolNode,
 )
 from mypy.state import state
@@ -3402,3 +3404,29 @@ def callable_with_ellipsis(any_type: AnyType, ret_type: Type, fallback: Instance
         fallback=fallback,
         is_ellipsis_args=True,
     )
+
+
+def store_argument_type(
+    defn: FuncItem, i: int, typ: CallableType, named_type: Callable[[str, list[Type]], Instance]
+) -> None:
+    arg_type = typ.arg_types[i]
+    if typ.arg_kinds[i] == ARG_STAR:
+        if isinstance(arg_type, ParamSpecType):
+            pass
+        elif isinstance(arg_type, UnpackType):
+            if isinstance(get_proper_type(arg_type.type), TupleType):
+                # Instead of using Tuple[Unpack[Tuple[...]]], just use
+                # Tuple[...]
+                arg_type = arg_type.type
+            else:
+                arg_type = TupleType(
+                    [arg_type],
+                    fallback=named_type("builtins.tuple", [named_type("builtins.object", [])]),
+                )
+        else:
+            # builtins.tuple[T] is typing.Tuple[T, ...]
+            arg_type = named_type("builtins.tuple", [arg_type])
+    elif typ.arg_kinds[i] == ARG_STAR2:
+        if not isinstance(arg_type, ParamSpecType) and not typ.unpack_kwargs:
+            arg_type = named_type("builtins.dict", [named_type("builtins.str", []), arg_type])
+    defn.arguments[i].variable.type = arg_type

test-data/unit/check-classes.test

@@ -7664,3 +7664,18 @@ class C(B):
     def foo(self) -> int:  # E: Signature of "foo" incompatible with supertype "B"
         ...
 [builtins fixtures/property.pyi]
+
+[case testAllowArgumentAsBaseClass]
+from typing import Any, Type
+
+def e(b) -> None:
+    class D(b): ...
+
+def f(b: Any) -> None:
+    class D(b): ...
+
+def g(b: Type[Any]) -> None:
+    class D(b): ...
+
+def h(b: type) -> None:
+    class D(b): ...

test-data/unit/check-python38.test

@@ -718,3 +718,19 @@ def f1() -> None:
         y = x
     z = x
 [builtins fixtures/dict.pyi]
+
+[case testNarrowOnSelfInGeneric]
+# flags: --strict-optional
+from typing import Generic, TypeVar, Optional
+
+T = TypeVar("T", int, str)
+
+class C(Generic[T]):
+    x: Optional[T]
+    def meth(self) -> Optional[T]:
+        if (y := self.x) is not None:
+            reveal_type(y)
+        return None
+[out]
+main:10: note: Revealed type is "builtins.int"
+main:10: note: Revealed type is "builtins.str"

test-data/unit/check-selftype.test

@@ -1772,3 +1772,16 @@ class D(C): ...
 
 reveal_type(D.f)  # N: Revealed type is "def [T] (T`-1) -> T`-1"
 reveal_type(D().f)  # N: Revealed type is "def () -> __main__.D"
+
+[case testTypingSelfOnSuperTypeVarValues]
+from typing import Self, Generic, TypeVar
+
+T = TypeVar("T", int, str)
+
+class B:
+    def copy(self) -> Self: ...
+class C(B, Generic[T]):
+    def copy(self) -> Self:
+        inst = super().copy()
+        reveal_type(inst)  # N: Revealed type is "Self`0"
+        return inst

test-data/unit/semanal-types.test

@@ -790,6 +790,7 @@ def f(x: int) -> None: pass
 def f(*args) -> None: pass
 
 x = f
+[builtins fixtures/tuple.pyi]
 [out]
 MypyFile:1(
   ImportFrom:1(typing, [overload])
@@ -1032,6 +1033,7 @@ MypyFile:1(
 
 [case testVarArgsAndKeywordArgs]
 def g(*x: int, y: str = ''): pass
+[builtins fixtures/tuple.pyi]
 [out]
 MypyFile:1(
   FuncDef:1(